1. Field of the Invention
The present invention is directed to a method for generating a resultant tomogram from a number of tomograms registered with a computed tomography (CT) apparatus by scanning slices of an examination subject that are at different, fixed positions during the scan on a system axis proceeding at a right angle relative to the slices, i.e. what are referred to as transverse slices.
2. Description of the Prior Art
The registration of a sequence of tomograms is a standard technique with great significance in computed tomography. Due to the feed of the examination subject relative to the measuring unit that occurs between the individual tomograms, the individual tomograms are allocated to different z-positions, the z-coordinate indicating the relative position of the slice of the examination subject imaged in the tomogram with respect to the z-direction residing at a right angle relative to the slice plane.
Without further measures, it is not possible to acquire tomograms for z-positions other than those allocated to the registered tomograms. It is also not possible to select the effective slice thickness belonging to the respective nomogram, i.e. the half-width value of the slice sensitivity profile belonging to the tomogram, deviating from the collimated slice thickness this is set by diaphragms, i.e. the expanse in z-direction of the X-ray beam employed for the registration of the tomogram.
Therefore, in known methods of the type initially described, the tomograms are simply calculated from the measured data generated in the scan without influencing the effective slice thickness or the z-position of the individual tomograms. This is considered as disadvantageous in practice for the following reasons:
When subjects, for example organs, or parts thereof project only partially into the slice of the examination subject to be imaged in the respective tomogram, a partial volume artifact arises. It is expressed in a modification of the measured data characterizing the respective subject or subject part and its environment; the contour of the subject or subject part itself also can be changed. Partial volume artifacts become more frequent as the collimated slice thickness becomes larger. Although a reduction of the collimated slice thickness reduces the occurrence of partial volume artifacts, it simultaneously increases the noise amplitude.
If diagnostically relevant regions of an examination subject are to be imaged with different effective slice thicknesses, then a number of sequences with different slice thicknesses must be registered , causing undesirably increased radiation stress for the examination subject.
The measured data acquired during the course of a sequence allow the reconstruction of tomograms only for those z-positions for which tomograms were in fact registered during the sequence. If it turns out later that tomograms of deviating z-positions would be helpful, then these tomograms must be additionally registered, which likewise means an additional radiation stress for the examination subject.
In order to avoid these disadvantages at least to a certain extent, it is known to acquire a resultant tomogram from a number of tomograms of a sequence by forming an average value. For example in that a resultant tomogram having the effective slice thickness nd is calculated from n adjoining tomograms having the collimated slice thickness d. However, the reconstruction time for n individual tomograms is required for the calculation of such a resultant tomogram since these all must be available before the averaging. Such a multiplication of the calculating time is ultimately prohibitive for the described procedure.
German OS 196 25 863 and Crawford et al, xe2x80x9cComputed Tomography Scanning With Simultaneous Patient Translation,xe2x80x9d Med. Phys. 17(6), November/December 1990, pages 967-982, disclose determining the data that belong to a slice exhibiting a specific position on the system axis during the course of the image reconstruction in spiral scanning on the basis of spiral interpolation. The data belonging to the slice to be reconstructed are acquired for the individual projection angles by an interpolation between data that exhibit the respective projection angle but have positions on the system axis that deviate from the position of the slice.
An object of the present invention is to provide a method of the type initially described that allows the generation of a resultant tomogram without the disadvantage of multiplication of the calculating time.
The above object is achieved in a method for generating a resultant tomogram from a number of tomograms registered with a computed tomography apparatus in accordance with the invention, by scanning slices of an examination subject that have different, fixed positions during the scan along a system axis which proceeds at a right angle relative to the slices. The attenuation values acquired in the scanning of the individual slices are superimposed to form resulting attenuation values and the resultant tomogram is reconstructed from these resulting attenuation values.
The system axis preferably but not necessarily proceeds at a right angle to the planes of the slices.
It is important for the invention that no superimposition of tomograms occurs, but, rather, the generation of the resultant tomogram ensues on the basis of the superimposition of attenuation values belonging to the tomograms to be superimposed to form resultant attenuation values which form the basis from which the resultant tomogram is reconstructed.
The calculating time required for the generation of the resultant tomogram is not significantly longer than the time required for the reconstruction of a single tomogram, since the determination of resultant attenuation values is less time-consuming compared to the reconstruction of a tomogram.
It is advantageous in the inventive method that an influencing of the noise amplitude of the resultant tomogram as well as of the effective slice thickness of the resultant tomogram is possible, namely by modification of at least one of the parameters Ns (number of slices involved in the superimposition), g(z) (weight with which the respective slice contributes to the result of the superimposition) and xcex94z (distance between two successive layers involved in the superimposition).
There is also the possibility of reducing the intensity of partial volume artifacts.
The radiation exposition and the dwell time of the patient in the CT apparatus are reduced in the following applications, that have not been possible with the known procedures for the registration of sequences:
For investigations which require effective layer thicknesses of different sizes (for example, soft tissue and bone diagnostics in the same volume), there is the possibility of implementing bone diagnostics with tomograms that are reconstructed for the collimated slice thickness on the basis of the unmodified attenuation values. Although these have a higher noise amplitude due to the small collimated slice thickness, they are observed with a window width that is far greater than the noise amplitude. Soft tissue diagnostics can then be performed with resultant tomograms that are calculated, with the inventive method according to Equation (3) explained below, for example with Ns=5 and xcex94z=d, from resultant attenuation values calculated from attenuation values acquired in the course of the same sequence. This procedure is advantageous for the patient because the patient is not exposed to the radiation of a second scan with a larger collimated slice thickness and the patient need not remain in the CT apparatus for the duration of a second scan.
The same advantage is also achieved when effective slice thicknesses of different size are needed in volume regions adjacent to one another. The registration of the attenuation values for the entire volume covering adjacent volume regions can then be implemented in the course of one and the same sequence with a single (small) collimated slice thickness, and a second sequence with a different collimated slice thickness is completely eliminated. The effective slice thickness suitable for the respective volume region is then selected in the reconstruction of resultant tomograms.